concurrency.js

function fakeAjax (url, cb) {
  var fake_responses = {
    file1: 'The first text',
    file2: 'The middle text',
    file3: 'The last text'
  }
  var randomDelay = (Math.round(Math.random() * 1e4) % 8000) + 1000

  console.log('Requesting: ' + url)

  setTimeout(function () {
    cb(fake_responses[url])
  }, randomDelay)
}

function output (text) {
  console.log(text)
}

// **************************************
// The old-n-busted callback way

function getFile (file) {
  fakeAjax(file, function (text) {
    handleResponse(file, text)
  })
}

function handleResponse (filename, contents) {
  if (!(filename in responses)) {
    responses[filename] = contents
  }

  var filenames = ['file1', 'file2', 'file3']
  for (var i = 0; i < filenames.length; i++) {
    if (filenames[i] in responses) {
      if (typeof responses[filenames[i]] === 'string') {
        output(responses[filenames[i]])
        responses[filenames[i]] = false
      }
    } else {
      return false
    }
	}
	
	output("Complete!");
}

var responses = {}

// request all files at once in "parallel"
getFile('file1')
getFile('file2')
getFile('file3')